This invention relates to a method of treating anodic oxide films or coatings to provide thereto high heat radiance, heat resistance and electrical insulation, a printed wiring board using such anodic oxide film and a method of making the same.
Anodic oxide films formed on valve metals such as aluminum, titanium, magnesium, tantalum, and their alloys are electrically insulating metallic oxide films which are conversion-coated on their surfaces. The anodic oxide films have homogeneous composition and are excellent in adhesion to their base metal, corrosion resistance and thermal conductivity. By the use of these properties metals having anodic oxide films formed thereon are widely used for printed wiring boards of electronic instruments, radiator plates of electronic parts, electrically insulative heat sinks, insulated wires, building materials and the like. In order to enhance insulation and corrosion properties thick anodic oxide films are used for such use, but for the oxide films having a thickness of not less than several microns a great number of micropores are necessarily formed therein. The micropores are very small holes having a diameter of several hundred angstroms which reach the barrier layer of the anodic oxide film. Entrance of moisture in the air into and residue of anodizing electrolytic solution in the micropores make it impossible to obtain a thick anodic oxide film having sufficiently high electrical insulation and corrosion resistance. To avoid these drawbacks sealing treatment can be rendered on the micropores for sealing thereof by the use of steam or boiling water. However by such treatment the anodic oxide film is subjected to hydration and cracks are produced in thus hydrated film merely by heating it up to around 150.degree. C.
With respect to anodic oxide films subjected to no sealing treatment cracks occur therein by heating up to about 200.degree. C. The cracking is caused by thermal stresses in the anodic oxide film due to the difference in coefficient of thermal expansion between the film and its base metal. Cracked anodic oxide films are inferior in electrical insulation and corrosion resistance.
For the sealing of the micropores there has been proposed another treatment that the micropores are impregnated with resins and the like. However micropores of almite films, for instance, have a very small diameter of several hundred angstroms and a depth of several ten micrometers and hence it in practice impossible to impregnate the micropores with resins to their innermost portions because of viscosity thereof.
To solve this problem it has been proposed that electrodischarge is effected in an atmosphere of a gas of an organometallic compound such as organic silicon compound with an anodic oxide film made as one electrode, thereby depositing the organometallic compound in the micropores and on the surface of the oxide film. (Japanese Patent Publication (examined) or Koukoku Kouhou No. 49-4719). Since this treatment employs the electric discharge phenomenon, it is impossible to control the deposition position of the organometallic compound, so that it is difficult to sufficiently fill the micropores with the compound. Further the treatment has disadvantages that electric insulation property of the anodic oxide film is liable to be deteriorated due to moisture in the air since according to this treatment the organic metal compound cannot be compactly deposited and discharge current paths remain in the deposited compound.